1. Field of the Invention
The present invention pertains to restoration of teeth. More specifically, the present invention pertains to methods of forming porcelain tooth restorations for dental restoration of worn, damaged or malformed teeth.
2. Description of the Prior Art
There are many methods of restoring or repairing teeth. For several decades, practitioners in the field of restorative dentistry have developed several methods and materials for restoring worn, damaged or malformed teeth with porcelain restorations. Porcelain is attractive and relatively inexpensive. Porcelain restorations may be used for inlays, onlays, crowns and veneers to correct structural and/or cosmetic deficiency of teeth. Such porcelain restorations are custom made for bonding to an underlying or adjacent tooth structure.
In most recent times, there have been two basic methods for producing an all-porcelain restoration: the foil or "indirect" method and the refractory or "direct" method. In the foil method, a mold is made of the tooth structure on which the restoration is to be placed and a positive replication of the tooth structure is formed from a material poured into the mold. Then a platinum foil matrix is applied to and burnished over the tooth structure replication. Then, dental porcelain, in a water-based slurry, is applied over the foil matrix, baked in a furnace, ground and glazed to produce a restoration which can be bonded to the original tooth structure.
Though the foil method of producing a porcelain restoration has been proven, there are problems associated with such a method. Foil, by its nature, is difficult to completely form and adapt to the surface of a tooth structure replication and due to the fact that porcelain must be built up on the foil and must be removed from the tooth replication for subsequent firings and with the final peeling out of the foil from the finished restoration, the porcelain restorations frequently are deficient in accuracy of fit. This requires filling in with other materials so that the tooth restoration may be bonded to the original tooth structure.
The second and more accurate method of producing porcelain restorations requires the use of high heat resistant refractory investment materials molded in the shape of the tooth structure on which the toot restoration is to be placed. The refractory investment replicates the original tooth structure and allows for direct application and subsequent firings of porcelain thereto. This method, referred to as the refractory or "direct" method results in a porcelain restoration with far greater accuracy of fit.
Even though the direct method of producing a porcelain restoration is preferred in the dental industry, there are a number of problems and inconveniences associated therewith. In typical practice, the tooth structure to which the porcelain is to be bonded is replicated with a high heat resistant refractory investment material. The tooth structure replication must undergo a degassing or curing process. Then, before a water-based porcelain mixture can be applied to the tooth structure replication, water must be absorbed into the refractory. If this is not done, the water in the water-based porcelain slurry will be quickly absorbed into the refractory, preventing the porcelain from flowing or being adapted to the surface of the tooth replication.
Even with careful saturation of the refractory tooth replication, problems occur. For example, in U.S. Pat. No. 4,579,530 it is mentioned that the first application of porcelain slurry, after being cured, produces a very cracked surface having, as described in the above referenced patent, the appearance of "a dried-out river bed". This appears to be due to the absorption of water into the refractory tooth replication and subsequent drying thereof. These cracked surfaces then require healing by adding succeeding layers of porcelain.
In addition to resulting in a very cracked surface, the presently used direct method of porcelain tooth restoration requires considerable waiting time (at least four to ten minutes) for water absorption by the tooth replication refractory. Porcelain lifting or peeling problems are also encountered in firing subsequent layers of porcelain. Thus, while this method of forming porcelain restorations is one of the most effective to date, there are improvements which need to be made.